A variety of aqueous streams, either process or waste streams, contain metal-cyanide complexes that are not easily treated to remove cyanide. Included in the various streams containing metal-cyanide complexes are aqueous electroplating and metal finishing solutions and rinse waters, aqueous processing streams from mineral leaching operations, and aqueous processing streams from the photographic industry. The annual production of cyanide-containing hazardous wastes is estimated at 5 to 6 billion gallons in the 1990's. The contaminated solid waste is considered environmentally hazardous, and the presence of the cyanide results in a significant increase in sludge disposal cost for the metal finishing industry, the mining industry and any other industry that produces metal-cyanide containing wastes.
Metal-cyanide complexes formed in these industries including metals such as iron, nickel, zinc, cobalt, cadmium, copper, mercury, and precious metals such as silver, gold, platinum and palladium are generally such strong complexes that treatment with dilute mineral acid does not readily release the cyanide ions from the metal without forcing the process, for example, by continuously removing hydrogen cyanide from the solution, by using high concentrations of mineral acid, or by strong precipitation agents such as sulfides.
Iron, for example, binds so strongly to cyanide that treatment with strong acid, even when heated for long periods of time, does not completely separate the cyanide ions from the ferric ions. Ferricyanide is a very stable species, as iron(III) has one of the highest stability constants (.beta..sub.6 of Fe(CN).sub.6.sup.3- =43.6) of any metal typically found in many of the waste streams. Zinc and cadmium will release cyanide ions and form hydrogen cyanide upon treatment with 10% sulfuric acid, while copper under goes this reaction only slowly.
Nor does a commonly used cyanide destruction technique, such as treatment with oxidizers such as alkaline chlorination (hypochlorite under basic conditions), easily destroy all the cyanide when it is bound to such metal ions. For example, iron, cobalt and nickel cyanides are not effected by basic hypochlorite treatment and are often precipitated in the sludge that is formed under the basic conditions of the process. In general, many treatment processes for cyanide destruction are carried out under basic conditions (pH&gt;8) to prevent the release of any hazardous gaseous hydrogen cyanide. Thus, elevated levels of complexed cyanide can typically appear in hydroxide precipitated heavy-metal sludges produced during the treatment of many electroplating waste water solutions.
Other cyanide-containing waste streams include mineral processing streams. Cyanide is widely used in leaching precious metal ores, and is sometimes used in flotation processes for minerals. For over 50 years, cyanide leaching has been the method of choice for dissolving finely disseminated gold and silver from their ores. In typical commercial operations, dilute caustic cyanide solution is percolated through crushed or ground ore to dissolve the precious metals. This leaves a waste solution that contains some of the original free cyanide along with complexed cyanides of base metals such as iron(III) and copper(II) and of toxic metals such as mercury(II). Some ores are so high in iron that this represents a significant hazardous waste stream as there are no acceptable chemical processes for displacing cyanide from iron. Currently, these waste metal-cyanide complexes are often placed in holding ponds with the hopes that environmental and biological degradation will ultimately destroy the cyanide. Since bacterial destruction is a slow process, these exposed ponds pose severe environmental hazards to migrating animals or to potential overflow from winter runoff.
In some of these processes it is desirable to destroy the cyanide after use, in others, it is desirable to recover the cyanide for further use. It might be desirable to be able to readily recover cyanide from all these types of metal-cyanide containing waste streams from both materials and cost savings consideration. From an environmental perspective cyanide should be either recovered or fully destroyed and not be allowed to exist in landfills and ponds where it could be potentially released by the leaching action of the environment. Likewise, in some cases metal value is wasted and the metal ions that are deposited in landfills are wasted resources.
Water-soluble polymers are well known for the retention or recovery of certain metal ions from solutions under certain conditions, e.g., certain pH conditions (see, e.g., Talanta, vol. 36, No. 8, pp. 861-863 (1989) and U.S. Pat. No. 4,741,831). Additionally, higher molecular weight varieties of the water-soluble polymers such as polyethyleneimine have been used as coatings on, e.g., silica gel, for separation and recovery of metal ions. However, there has not previously been any use of a water-soluble polymer for the displacement or separation of cyanide from metal-cyanide complexes.
It has now been found that by choosing the appropriate water-soluble polymer including selected chelating groups, i.e., functionalities, that the following displacement reaction of cyanide can occur in solution: EQU M(CN.sup.-).sub.x +PH.fwdarw.M(P)+xCN.sup.- +H.sup.+ ; EQU M(CN.sup.-).sub.x +PNa.fwdarw.M(P)+xCN.sup.- +Na.sup.+ ; or EQU M(CN.sup.-).sub.x +P.fwdarw.M(P)+xCN.sup.-.
where M is the metal ion, CN.sup.- is the cyanide ion, x is the number of cyanides present in the complex, H.sup.+ is a proton, Na.sup.+ is a sodium ion, and PH (or PNa) is the water-soluble polymer or its sodium salt. In PH or PNa, the H or Na would be an acidic proton or the sodium salt thereof on a functional group such as a carboxylic group on the polymer. The complex, M(CN.sup.-).sub.x, may actually have a negative charge or be neutral. When the chemical reaction is accompanied with physical separation by ultrafiltration, the polymer-metal complex is retained and the free cyanide ion passes freely through the ultrafiltration membrane. The cyanide can be recovered (see, e.g., U.S. Pat. No. 4,895,659), reused, or destroyed as desired using a variety of oxidation techniques.
The metal ions can be released from the polymer and either recovered for reuse or precipitated and solidified for proper waste management. The following equations describe some of the possible ways to recover the metal ion from the polymer: EQU M(P)+(H.sup.+ .fwdarw.H(P)+M.sup.+ EQU M(P)+L.fwdarw.ML+(P)
or EQU M.sup.x (P)+e.sup.- .fwdarw.M.sup.x-1 +(P)
where M is the metal ion, (P) is the water-soluble polymer, L is a competing complexant, H.sup.+ is a proton from an acid, x is the valent state of the metal, and e.sup.- is an electron for an oxidation change reaction.
Therefore, it is an object of the present invention to provide a process of displacing cyanide ion from metal-cyanide complexes by use of suitable water-soluble polymers prior to a hydroxide precipitation stage or other waste management stage.
It is a further object of the present invention to provide a process of separating the metal ion from metal-cyanide complexes by use of suitable water-soluble polymers prior to a hydroxide precipitation stage or other waste management stage.
It is a still further object of the invention to provide a process of reducing the carry-over of cyanide into hydroxide precipitated sludges.
It is a still further object of the invention to provide a process of recovering cyanide for reuse from selected processes.